MILK 



45 



top, the sample tubes are centrifuged in a suitable centrifuge 

 (e.g. Gerber's) for three minutes, and a clear ethereal solution is 

 obtained. Finally, the tubes are placed in a water-bath having 

 a temperature of 17'5 C. 



Before beginning the determination by the refractometer, that 

 instrument must be adjusted. This is done by putting a drop 



JM<;. 15. Apparatus for use in Wollny's Method. 



of pure distilled water (temp. 17*5 C.) between the prisms, the 

 water being introduced through the narrow canal which leads 

 from the outside to the space between the faces of the prisms. 

 If the instrument is correct and the temperature 17*5 C., the 

 boundary line between the light and dark field should fall upon 

 the zero of the scale. After taking this reading, the lower prism 

 is thrown back, and the surface of each prism carefully dried 

 with a fine linen cloth. In a similar way a test is made 



